Many parts are produced by the method of die casting. In this method, molten material is added to a die and cooled to form the part. The die may be a one time use type or a reusable type. Often, reusable dies are provided with means for cooling the die. Especially when larger parts are produced, the associated die includes passages through which a cooling medium is passed. Cooling the die not only maintains the appropriate size and shape of the die during casting, but helps control the cooling rate of the cast part.
Cooling the die is an issue of heat balancing. Heat is provided by the molten material added to the die. Heat is removed by the cooling medium circulated through the die. Providing too much cooling through the die can result in the formation of inferior products because the molten material cannot effectively reach into all of the sections of the die. Additionally or alternatively, rapid cooling of the molten material can cause porosity or cracks in the finished part. On the other hand, if too much heat is provided, the die can expand and alter its shape, producing inferior parts. At extreme head loads, the die can crack or otherwise fail.
Although continuous operation of the die casting process is preferable, for various reasons production must be interrupted periodically. When this occurs, typically a valve which controls regular flow to the die in a principal cooling loop is closed and a bypass cooling loop is implemented. The bypass loop includes an adjustable manual valve and provides approximately one half the normal volume of cooling fluid to the die. When production is restarted, it is necessary to bring the die up to an appropriate temperature again. This is done by forming “preheat shots” within the die. These are castings which are discarded. In this process, molten material is poured into the die, thus heating the die, but the die is still too cold for these “preheat shots” to be considered parts meeting required quality standards. Typically, the formation of about six preheat shots is required to bring the die back to an acceptable temperature to produce parts meeting quality standards. As these “preheat shots” are considered to be scrap, a method to reduce the number required to be formed is desirable.